1. Field of the Invention
The present invention relates to an imaging apparatus with a folded zoom optical system and a method of driving its imaging optical system.
2. Description of Related Art
In recent years, a so-called folded optical system has been used generally for the optical system of an imaging apparatus. The folded optical system employs, in order to guide a luminous flux incident from a subject to an imaging element such as a CCD, the configuration of folding the optical axis by means of an optical reflective element such as a prism. The adoption of such a folded optical system allows the flexibility of the optical system layout in an imaging apparatus to be increased as compared with normal optical systems without folding the optical axis, and therefore can contribute to make an imaging apparatus smaller and thinner.
Further, since there is no need to advance a lens barrel from an imaging apparatus, a digital camera employing such a folded zoom optical system has been proposed, which is for speeding up the starting and the ending operations (see JP2004-251937 A, for example). The specific configuration follows. When the shooting ends, the position of a movable lens is not adjusted but the movable lens is allowed to stop at that position, and the position is stored. Then, when the shooting is started, based on the stored position information of the movable lens, the control is started from the state before the end of the shooting. With this configuration, the time required for adjusting the position of the movable lens can be saved, and therefore the processing can be speeded up.
However, the actual products employing the folded zoom optical system as described in JP 2004-251937 A that does not require the lens barrel to advance have the 3× magnification at most, and cannot realize a large magnification of about 10× using the product ready for a high pixel counter such as 4-megapixel or 5-megapixel that has become mainstream recently. This is because a high performance is required also for the optical system in order to realize such a higher pixel counter and a larger magnification, leading to the necessity to increase the diameter of a lens closer to the object side, and this would degrade the effect of a thinner imaging apparatus, which is one of the advantages of the folded optical system. Then, in order to cope with such a problem and realize a thin imaging apparatus while allowing for a higher pixel counter and a larger magnification, the configuration as shown in FIG. 17 can be considered. That is, in a folded zoom optical system including a plurality of lens groups (e.g., first to fifth lens groups G1 to G5 arranged in this order from the object (subject) side to the image surface side), a prism P as a reflective optical element is disposed not in the first lens group G1 but in the second lens group G2, and the first lens group G1 whose lens diameter has to be increased is disposed so that its optical axis (a first optical axis A1) can be in parallel with the thickness direction of the imaging apparatus 1. During shooting (during operation), a collapsible lens barrel portion 2 is advanced as in the normal collapsible lens barrel. Then, the second lens group G2 or later, whose lens diameters are relatively small, are disposed in the direction perpendicular to the thickness direction of the imaging apparatus 1. Thereby, the overall thickness of the imaging apparatus 1 can be made smaller when it is not in use.
However, when such a configuration is employed, the first lens group G1 needs to perform the collapsing operation. Therefore, the ideas of high-speed starting and ending operations as described in JP 2004-251937 A cannot be incorporated, and it will take much time to perform the starting and the ending operations. Especially, when it takes a long time to perform the starting operation of the imaging apparatus, a good opportunity for a photograph will be missed.